U.S. Pat. No. 6,906,474 B2 (Trestman et al.) discloses an electronic ballast for powering one or more gas discharge lamps from a three-phase alternating current (AC) voltage source. U.S. Pat. No. 6,906,474 B2 teaches that, in comparison with conventional single-phase electronic ballasts, a three-phase electronic ballast provides significant advantages such as increased energy efficiency and improved reliability.
An essential component of a three-phase electronic ballast is an electromagnetic interference (EMI) filter that provides attenuation of high frequency noise generated by operation of a high frequency inverter. The level of attenuation that must be provided by the EMI filter is dictated by the standards established by certain regulatory agencies [e.g., the Federal Communications Commission (FCC) in the United States, and similar agencies in other countries].
The noise that is generated due to operation of a high frequency inverter consists of two components—a common-mode noise component and a differential-mode noise component. Of those two components, the common-mode component is typically prevalent. To effectively attenuate both the common-mode and differential-mode noise components, as well as to prevent the common-mode noise component from reaching the AC line source, it is necessary to have an EMI filter that fulfills the following requirements: (1) a filter inductor having a high impedance; and (2) low intrawinding capacitances (i.e., the parasitic capacitance that typically exists between layers of an inductor winding and that effectively create a circuit path which allow high frequency noise to bypass the inductor); and (3) sufficient X-capacitances, in order to effectively attenuate the differential-mode noise component; and (4) sufficient Y-capacitance, in order to provide a low impedance return path for the common-mode noise component that is injected into the ground wire by operation of the high frequency inverter.
FIG. 1 describes a preferred embodiment of a three-phase electronic ballast 10 disclosed in U.S. Pat. No. 6,906,474 B2. As depicted in FIG. 1, EMI filter 100 comprises three differential mode type inductors 102,104,106, two X-capacitances 112,114, and one Y-capacitance 120. In a practical realization of ballast 10, each X-capacitance must be sized (i.e., in terms of voltage rating) to withstand the maximum phase-to-phase voltage provided by three-phase AC voltage source 30. In the case of a three-phase AC voltage source having a nominal phase-to-phase voltage of 480 volts (rms), and in view of a possible variation (from nominal) of up to +/−10%, each of X-capacitances 112,114 must be capable of withstanding at least 528 volts (rms), which dictates that X-capacitances 112,114 must be realized using X-capacitors rated at 600 VAC. However, because standard 600 VAC rated X-capacitors are not currently available, each of X-capacitances 112,114 must be realized using a series combination of two 300 VAC rated X-capacitors. Thus, EMI filter 100 requires a total of four individual X-capacitors (each having a 300 VAC rating) in order to implement the two X-capacitances 112,114. This requirement detracts from the cost-effectiveness of implementing EMI filter 100.
Thus, a need exists for a three-phase ballast having a three-phase EMI filter that is capable of being realized in a more cost-effective manner than the prior art approach. A further need exists for a three-phase EMI filter that provides improved EMI suppression and/or added safety benefits over the prior art approach. Such a ballast and EMI filter would represent a significant advance over the prior art.